Many portable information devices have been used in recent years. Some of those portable information devices, such as smart phones, personal digital assistants (PDAs), and tablet devices, include touch panels as input means. The touch panel generally occupies almost all the front face of a device. The touch panel is made of a transparent substrate that transmits the light sent from a display device provided thereunder. Under the periphery of the touch panel surrounding the display device, a circuit board is disposed. It is not preferred that the circuit board is observable through the transparent touch panel. To prevent the circuit board from being observable, a light shield is provided on the periphery of the transparent substrate of the touch panel, which surrounds the display device.
For example, a frame punched from a thin metal plate or a light-shielding resin plate is bonded to the periphery of the transparent substrate to shield light. PTL 1 and PTL 2 disclose methods for applying black paint on the periphery of the transparent substrate by screen printing or pad printing.
Since the shading layer is observable on the surface of the information device, shading layers with various designs have been required recently.
For example, a touch panel of a cellular phone provides visual information on the status of the phone by turning on and off the light source or changing the colors of light when the user touches the panel. A touch panel of an air conditioning system in a vehicle is usually black and indistinguishable from a central console, and provides various kinds of literal information on temperature or air conditioning, for example, when the user touches the panel.
The touch panel includes a shading layer and a black shading layer provided thereunder having an aperture pattern corresponding to the shape of an indication to be displayed. The black shading layer is illuminated from the back and generates a light-emitting pattern corresponding to the shape of the aperture pattern. The upper shading layer has any color, for example, black, white, or blue depending on the product requirements.
A black shading layer has a sufficient shading effect even if the layer has a small thickness formed by printing, for example. Thus, the light-emitting pattern projected through the aperture pattern in the black shading layer has a clear outline and good visibility. A white shading layer generally develops its whiteness by light scattering. Thus, the white shading layer should be thick to have a sufficient shading effect and develop its bright whiteness. If a black shading layer having an aperture pattern is disposed under such a white shading layer to indicate various types of information, the light emitted from a back light source and projected through the aperture pattern generates a light-emitting pattern that is blurred outside the aperture pattern. The light may diffuses several or more times widely than the aperture pattern in a dark place, which significantly impairs display quality.
To solve this problem, PTL 3 discloses a thin white shading layer having a high shading effect. The white shading layer is formed by ink-jet printing and its surface has a predetermined arithmetic average roughness. However, this technique cannot sufficiently reduce the blurring of a light-emitting pattern.